The placental isoenzyme of alkaline phosphatase is a cell surface protein frequently reexpressed in human tumors, such as ovarian cancer and seminomas, as compared with normal tissues. Our goal is to examine the usefulness of PLAP as a tumor marker by investigating the possibility that monoclonal antibodies binding PLAP may be useful for detection of the growth and metastasis of tumors that express the enzyme. Thus, we developed an experimental system where a PLAP-positive and PLAP-negative tumor were each grown in the same nude mouse. The mouse was then injected with radioactively-labeled F(ab)'2 fragments of a monoclonal antibody. The results with two separate monoclonal antibodies indicate that the monoclonal antibody localized in the PLAP-positive tumor 10 times or more often than it localized in the PLAP-negative tumor or in normal mouse tissues. The concentration ratios using PLAP as the tumor marker are similar to those obtained by researchers employing carcinoembryonic antigen (CEA) as the tumor marker and a monoclonal antibody specific for CEA. Therefore, PLAP would appear to be a useful marker for the immunodetection of certain tumors in humans by external scintigraphy. PLAP may similarly be effective as a target for the delivery of toxic reagents to tumor cells in vivo using drugs conjugated to PLAP-specific monoclonal antibodies. (AG)